The “last mile” is a phrase in telecommunications, cable television and internet industries relating to the connection of retail customers (e.g., homes or offices) to the pertinent network (e.g., the telephone network or the internet). The “last mile” connections, generally consist of copper twisted pairs, typically exhibit a bandwidth “bottleneck” limiting the rate of data delivery to the customers since twisted pairs where originally deployed to support voice signals, and not to communicate high bit rates typical to broadband Internet access. Furthermore, “last mile” connections are expensive to upgrade (e.g., to optical fibers) due to the large number of such connections (i.e., relative to the number of connections between exchanges or routers).
Reference is now made to FIG. 1, which is a schematic illustration of a typical “last mile” connection which is known in the art. Such a typical “last mile” connection includes a building 10 a distribution point (DP) 20 and a central office 24. Building 10 includes, for example, eight apartments 121-128. Each of apartments 121-128 includes, for example, a respective one of computers 141-148 coupled with a respective one of modems 161-168 either directly or via a router or hub (not shown). Each one of modems 161-168 is coupled with distribution point 20 via a respective one of line connections 181-188 also known as “drops”. Each one of line connections 181-188 is, for example, a twisted pair of wires. Each one of line connections 181-188 may further be, for example, a coaxial cable. Line connections 181-188 are grouped together within a binder 22. Distribution point 20 along with line connections 181-188 and computers 141-148 constitute a communication system. Distribution point 20 is coupled with Central office 24 via a communication channel 26 (e.g., optical fiber, cable, wireless channel). The distance between building 10 and distribution point 20 is up to the order of hundreds of meters and typically up to 200 meters. The distance between distribution point 20 and central office 24 is up to the order of several kilometers.
It is noted that computers 141-148 are brought herein as an example only. Other devices require communication services (e.g., smart TV's, smartphones, IP phones, routers) may be coupled with the respective one of modems 161-168. Furthermore, building 10 may include offices rather than apartments. Additionally, the number of apartments or offices in building 10 may be different than eight (e.g. four, sixteen). Additionally, the distribution point may be connected to a plurality of private homes.
Data transmission includes downstream transmission of data from the DP toward the CPE also referred to as downlink (DL). Data transmission also includes upstream transmission of data from the CPE toward the DP also referred to as uplink (UL). Furthermore, data transmission is divided into data frames, where each frame includes a plurality of time-slots each for transmitting a data symbols (i.e., a combination of bits, which is encoded and modulated to create the data symbol exhibiting the duration of each time-slot). Nevertheless, the terms ‘time-slot’ and ‘symbol’ are used herein interchangeably. In each frame, a portion of the symbols may be designated for downlink transmission and a portion of the symbols may be designated for uplink transmission. Frames may further be grouped in super-frames, where each super-frame includes a plurality (e.g., on the order of tens) of frames. Reference is now made to FIG. 2 which is a schematic of a super-frame, generally referenced 50, which is known in the art. Super-frame 50 includes a plurality of frames. The duration of super-frame 50 may be on the order of several milliseconds (ms) and typically 6 ms and each frame typically includes between 20 symbols and 40 symbols. Each frame, for example, frame 52, which corresponds to the second frame of super-frame 50, includes a plurality of time-slots, such as time-slot 54 for transmission of data symbols.
“G.fast” technology attempts to increase the data rate between the distribution point and the Customer Premise Equipment (CPE—such as modems, routers, hubs, computers, Smart TV's and the like) to the order of one Giga bits per second (i.e., 1 Gbps). Typically, the bandwidth of each twisted pair is between 100-200 Megahertz (MHz) and the number of twisted pairs per binder is between eight and sixteen. As a result of the high frequencies employed, a high degree of cross-talk interference exists between the different twisted pairs in the binder. In essence, due to the high level of cross-talk, the coupling between the distribution point and different CPE's may be considered as a multiple access problem where a plurality of devices are coupled with the plurality of CPE's. Such a coupling or channel may be described in a matrix form where the entries in the matrix represent the different coupling factors. Cancelling cross-talk interference is also referred to as “vectoring”. Vectoring means the use of one or both of precoding in the downlink direction and cross-talk cancellation in the uplink direction. In general, power consumption is of utmost importance for system implementations designed to be installed in the distribution point.
In general, two primary parameters are associated with each user, the line capacity (i.e., the achievable bit rate of the line assuming continues transmission) and the service rate (i.e., the bit rate that the customer is subscribed to or that the service provider is committed to provide). These two parameters may be different one with respect to the other. Each customer may be subscribed to a different service rate. Each of the different lines connected to the Distribution Point may also have different capacities due to the differences in the distance from the DP, differences in the home wiring, differences in the lines attenuation and cross-talk within the binder (common in the high frequencies) and differences in the background noise levels. The differences in the line capacities and the service rates translate to variations in the required transmission durations for each line. The transmission duty-cycle is determined according to the ratio between the service bit-rate and the line capacity. For example, if the service bit-rate is 100 Mbps and the line capacity is 200 Mbps we will have to transmit for approximately 50% of the time (i.e., ignoring the gaps and overheads). In general, transmission duty-cycles may vary between say 10% (1 Gbps line capacity, 100 Mbps service) to 100% (100 Mbps line capacity, 100 Mbps service).
U.S. Pat. No. 7,817,745 to Cioffi et al entitled “Tonal Precoding” describes therein, a Digital Subscriber Line (DSL) communication system which employs Discrete Multi-Tone (DMT) transmission and precoding in which U transmitters of U users are connected with U receivers employing Frequency Division Duplexing (FDD). The channel from the U transmitters to their U receivers can be modeled by a matrix channel H, whose size usually is U×U. The channel H can be decomposed into H=RQ using RQ factorization of a square matrix where the Q matrix can be used as a linear filter and R matrix can be used as a feedback filter for alleviating cross-talk. In one embodiment directed to by Cioffi et al, a G matrix in R=SG (where S is a diagonal scaling matrix that forces the diagonal elements of the triangular G to be all ones) can be interpreted as a set of precoder coefficients for the U users. These precoder coefficients can vary with each tone used by each user and depend upon the encoding order of users selected for each tone. In practice, the channel H is variable and the R and Q matrices are updated to adapt to such variability.
One of the embodiments directed to by Cioffi et al, employs adaptive precoding. Adaptive precoding adapts the precoding elements (e.g., the R, Q matrices, precoding coefficients, etc.) to changing channel or noise conditions or to both. In the adaptive system directed to by Cioffi et al, either the matrix R or the matrix Q or both may be updated by a controller as frequently as needed to match the time-variations of the channel, as well as the noise. Such updating may be triggered directly (e.g., by changes to the channel matrix H or to the noise spatial correlation Rnn), or indirectly (e.g., by changes to the bit tables or to the gain tables of the users of the vectored DSL system, or by changes to the precoding order within a tone, for the users of the vectored DSL system). The precoder may be updated after one or more of the receivers request new settings for the bit and gain tables of the vectored DSL transmitters. The precoder may also be updated after one or more of the users of the vectored DSL system are turned off, or after one or more users are added to the vectored DSL system.
U.S. Pat. No. 7,058,833 to Bremer et al, entitled “System and Method for Minimized Power Consumption for Frame and Cell Data Transmission Systems” directs to a transmitter power manager for reducing power in a communication system in which a communication device which is coupled with a plurality of communication connections. The transmitter power manager, which resides in the transmitter located in the central office, employs a data detector and power enabling circuitry. The data detector detects activity corresponding to an incoming communication signal which is to be transmitted. When such activity is detected the data detector provides a control signal to power enabling circuitry. The power enabling circuitry is coupled to selected elements in the transmitter and with the transmitter signal generating circuitry which may be powered down during periods of inactivity. When a control signal is received, which indicates the presence of an incoming communication signal which is to be transmitted, the power enabling circuitry provides power to the selected elements so that the selected elements are fully powered and ready to transmit the communication signal. When the communication signal has been transmitted, the data detector detects the end of the transmission and provides a control signal to the power enabling circuitry such that the selected components are then powered down. That is, during periods of transmitter inactivity, selected elements residing in the transmitter or the transmitter signal generating circuitry are powered down. When incoming communication signal, which are to be transmitted are detected, the power enabling circuitry turns on the selected elements.